What happened
On January 1, 2023, a Cessna 206, registration HK-2659, was conducting an air ambulance flight from La Primavera to Puerto Carreño, Colombia. Approximately five minutes after takeoff, the pilot experienced intense, rapid longitudinal oscillations in the control column, accompanied by a tendency for the aircraft's nose to pitch down. Simultaneously, the crew reported a loud, metallic noise coming from the rear of the aircraft near the right elevator.
Due to the loss of effective flight control, the pilot aborted the climb at approximately 2,500 feet and selected an unprepared field for an emergency landing. During the landing roll, the aircraft struck several termite mounds. The impact with a mound caused the nose gear to collapse and the aircraft to overturn 180 degrees. All three occupants (the pilot, a physician, and a nurse) sustained minor injuries and evacuated the aircraft on their own.
The investigation
The GRIAA investigation focused on the structural integrity of the elevator assembly and the maintenance history of the aircraft. Investigators examined the bracket assembly (P/N: 1234009-6) and identified that the trim tab had become uncoupled from the right elevator. The investigation also reviewed the operator's maintenance manuals and the manufacturer's inspection protocols regarding non-destructive testing for control surface components.
Findings
- The primary cause of the flight instability was the uncoupling of the elevator trim tab due to a ductile fracture of the bracket assembly, which resulted from differential aeration corrosion.
- Maintenance errors contributed to the failure; specifically, during a previous major service, the bracket assembly was improperly coated with paint, leaving the metal vulnerable to oxidation.
- The aircraft's nose gear detached and the aircraft overturned during the emergency landing due to the impact with natural obstacles (termite mounds) on the unprepared field.
- The operator's maintenance manual lacked specific inspection procedures or life-limit limitations for these critical components.
- There was a lack of detailed non-destructive testing (NDT) procedures in the manufacturer's manual to ensure the continued integrity of such brackets.